Nose-to-rider is approximately 3-4', depending on how far forward the horse's neck is stretched.
Also, I need to recheck my measurements, as it's supposedly 45' from the start line to the barrel I was shooting, and I was about 5-6' back from that. This would put the lights back another 30-40' from where I was. I didn't think it was that far. I'll pace it off this Friday to get a better distance approximation.
So, it looks like the DOF calculation may be correct, but the distance from the lights to the barrel is greater.
I shoot at barrel number two, from the left side of that barrel. The images there were shot about 10' from the time-line at the night race, and 6' from the time line during the day. I'm lined up approximately even with the green arrow that is to the left of the barrel in the diagram. If I had to hazard a guess, perhaps 20 degrees off perpendicular to the entry-side/bottom (as you view it here) of the barrel.
In doing the DOF thing with a 50' distance, the 1Dx calculates to be about 8.09' front-to-back @ f/7.1 (3.7' front/4.3' behind)
However, I really don't get that much DOF. Since the focus point is the front of the barrel, and the "money" shot is of the rider just entering the barrel to about where the rider is AT the barrel, which would seem to make my "window" of proper DOF a lot less than the 8' it seems.
I am still trying to picture that, but if I need both rider and horse's head in focus, I will need to stop down further for more margin-of-error, since getting the exact same shot/DOF each time is nearly impossible, given the variances of how the horses do on each run. Make sense?
Again, thanks for all your help on this. It's much appreciated.